I had recently flew to Bangkok with China Eastern through Nanjing Airport. Though a visa is typically required to enter China (10 yr visas are available), a visa free transit policy was introduced a few years ago offering up to 144 hours depending on the city. See this link.
I emailed China Eastern to confirm Nanjing Lukou was visa free and was sent this link confirming visa free transit is available.
Nanjing Lukou NKG is a large airport but not very busy for it’s size. A sign indicates it does not have a transit area. Typically, airports will have a way to go from the arrival area directly to the departure area with screening for transit passengers.
On arrival, you need to indicate you are transitting without a visa to the customs officer. You do not need to fill out any paperwork and the officer will collect your passport and direct you to the waiting area with the other transitting passengers from your flight.
If you have a visa, you need to fill out a departure (yellow) form. If you want to leave the airport through the 144 hr visa free transit, you also need to fill out a blue form to apply for it.
Once all the passengers from your flight are processed, your group will be lead to pick up your luggage from baggage claim and then head upstairs to recheck your luggage back in. They will also print your boarding passes at this time. You will then be lead as a group to departures immigration where they will return your passport and you can go through security to the departures area. You still need to fill out a yellow departure form. Though the staff speak English, they do have difficulty pronouncing names so someone should volunteer to help them read off names.
There aren’t a lot of food options post-security. Just a noodle restaurant (accepts Visa credit card) and a Subway.
Update: As a bonus, I got mileage credit on Delta Skymiles because China Eastern is a partner airline. They only gave credit on half the flights so I had to make a request for the missing mileage.